uv-k5-v3-emulator/AGENTS.md

Working on this repo

Notes for whoever picks this up next. Focused on what is not obvious from the code, and on mistakes that already cost time here.

What this is

A QEMU machine for the Puya PY32F071 (Cortex-M0+), so Quansheng UV-K5 V3 firmware runs on a PC. Boots to the main loop in ~5 s; the LCD is readable.

The machine and every device model live in one file, qemu/py32f071.c. That is deliberate: the models are small and tightly coupled to each other's wiring, and splitting them would spread the board layout out without making any of it clearer.

How it boots

Worth reading before debugging anything that looks like a startup problem. There is no bootloader, no kernel, no partition table and no filesystem -- the firmware is the only code on the machine and it owns the CPU outright.

The hardware knows two numbers. A Cortex-M0+ coming out of reset does not run any boot logic. It loads SP from the first word of the vector table and PC from the second, and starts executing. That is the whole handoff.

.isr_vector  0x08002800  (readelf -SW, size 0xc0)
  +0x00      0x20004000  initial SP, i.e. the top of the 16 KB SRAM
  +0x04      0x08002d49  Reset_Handler, and the ELF entry point

Read it straight off the image when in doubt -- the bytes are little-endian, so 00400020 492d0008 is SP 0x20004000 followed by PC 0x08002d49:

objdump -s -j .isr_vector firmware.elf | head -5

The odd address is not a typo: bit 0 flags Thumb state and the hardware masks it off when fetching.

PY32_APP_OFFSET 0x2800 is load-bearing. Flash starts at 0x08000000 but the first 10 KB is the factory bootloader region, so the application sits after it. armv7m_load_kernel() is passed that offset for exactly this reason -- load at 0x08000000 instead and the vector table lands in the wrong place, so the very first fetch faults.

Startup is 31 lines of assembly, in the firmware's Core/startup_py32f071xx.s:

set SP from _estack
bl SystemInit
copy .data from flash (_sidata) into RAM (_sdata .. _edata)
zero .bss (_sbss .. _ebss)
bl __libc_init_array
bl main
LoopForever: b LoopForever      @ main never returns

The copy and the zero-fill are the interesting part. Initialised globals live in flash but have to be writable, so they are copied word by word into RAM; uninitialised globals must read as zero per the C standard, so .bss is cleared. On a hosted OS the kernel and the loader do this for you. Here nobody does, so if either loop is wrong you get globals that are silently garbage.

Then the application:

main()                  Core/Src/main.c -- clock config only, then Main()
  Main()                App/main.c -- the actual firmware
    SYSTICK_Init()      the 10 ms tick everything is timed against
    BOARD_Init()        GPIO, SPI, LCD, keypad matrix
    UART_Init()         where the SERIAL banner in the log comes from
    SETTINGS_InitEEPROM()   reads settings over SPI from the flash image
    while (1) { ... }   main loop, never exits

There is no filesystem. The nearest thing to "mounting a partition" is SETTINGS_InitEEPROM() reading fixed byte offsets over SPI: 0xA008 for the power save byte, 0x0E70 for the VFO indices, and so on. No metadata, no directory, no checksum -- just an address that the code and the data both have to agree on. When a setting reads back wrong, suspect the offset before suspecting the transport.

The ~15 s to reach the main loop is emulation overhead. A real radio is up in about a second.

Ground rules

Never edit the firmware to make the emulator work. The firmware is the reference. If something does not run, the model is wrong. A fix that changes firmware source makes every later test meaningless, because you are no longer testing what the radio runs.

Register layouts come from the vendor CMSIS header, not from a datasheet search and not from inference:

<firmware>/Drivers/CMSIS/Device/PY32F071/Include/py32f071xB.h

When you need a bit position, read it from there. Several details are unintuitive — LL_ADC_FLAG_EOS is really ADC_SR_EOC on this part — and guessing produces models that look right and hang.

Find the next thing to model by watching where the firmware stops, not by reading the datasheet front to back. Every peripheral here was added because the firmware demonstrably waited on it:

tools/where.sh 4          # sample the call stack a few times

A stack that repeats in the same function across samples is a spin loop. Look at what it reads.

How to run it

python3 tools/make_flash.py    # once; builds assets/flash.img
tools/run.sh                  # GDB stub on :1234, QMP on /tmp/uvk5-qmp.sock

tools/where.sh                # where execution is
tools/gpiob_dump.sh           # GPIOB registers
python3 tools/key.py MENU     # inject a keypress
python3 tools/screenshot.py --frame-addr 0x200013DC \
    --status-addr 0x2000175C --port 1234 --out screen.png

Screenshot addresses move between firmware builds. Get the current ones with:

arm-none-eabi-nm firmware.elf | grep -E 'gFrameBuffer|gStatusLine'

Rebuild after editing the machine:

cd $QEMU/build && ninja qemu-system-arm    # ~10 s incremental

After any change near the keypad or the GPIO wiring, run the regression test. It boots its own instance on private ports, so it does not disturb a run.sh session:

python3 tools/keypad_test.py

There is also a browser UI, which is usually the quickest way to poke at the firmware by hand:

python3 tools/webui.py --frame-addr 0x200013DC \
    --status-addr 0x2000175C     # then open http://127.0.0.1:8080/

Two things about it that matter when working on this repo:

• It holds the QMP socket for its lifetime, so key.py cannot run at the same time. The socket accepts a single client.
• It reads frames with QMP memsave, deliberately. Not pmemsave, which takes a physical address and silently returns zeros for gFrameBuffer -- a blank screen with no error. And not gdb, which halts the guest on every attach: that stutters the stream and perturbs key debounce timing.

Its tests: tools/test_uvk5_*.py and tools/test_webui.py need no emulator, tools/test_webui_e2e.py boots its own.

The flash bugs: four faults, one symptom

"The frequency will not change" and "flash forgets everything after power off" looked like two complaints. They were one root cause plus three real bugs found on the way, all in this file. Worth reading before touching SPI, DMA or the flash model, because each was invisible from the layer above.

1. DMA used the wrong address space — the actual cause. It moved bytes through address_space_memory, which cannot decode this SoC's memory at all: the container region is handed only to the ARMv7M core and never registered with global system memory. Reads returned MEMTX_DECODE_ERROR and zeros; writes went nowhere. DMA now runs over an AddressSpace built on the container.
2. Page program did not wrap. Real SPI NOR latches only the low address bits, so a burst past the 256-byte page boundary continues at the start of the same page. The model walked straight through, and a 512-byte burst at 0x008F00 (which the firmware really does send in one CS assertion) spilled into 0x009000.
3. DMA started too early. Transfers ran when a channel was enabled, but on hardware they start when the peripheral raises its request. The driver arms both channels, then enables SPI, then sets TXDMAEN — so firing at arm time clocked the bus before the read command had been sent.
4. DMA channels ran one after another. SPI is duplex and the driver pairs a dummy-feeding TX channel with a data-collecting RX channel over one transfer. Running them in sequence let TX finish before RX ever sampled the bus.

Any one of them zeroed the sector holding per-band VFO frequencies. RADIO_ConfigureChannel substitutes a band's lower limit only for 0xFFFFFFFF, so a stored zero was taken literally and clamped to BX4819_band1_lower — 18 MHz. That is the whole explanation for a typed frequency always reverting.

tools/test_freq_entry.py and the MUST_NOT_CHANGE guard in tools/test_flash_persist.py exist to catch a regression in any of the four.

What made this hard to find, and what to do instead

Instrument the model, not the guest. The frequency input box times out after key_input_timeout_500ms / 3, about 2.5 s, and a gdb attach takes roughly 3 s. So probing between digits clears the box, and the run reports a failure that the measurement caused. This produced at least three confident wrong conclusions, including "the firmware saved band 0" when the box had simply emptied. Add an fprintf to qemu/py32f071.c and read stderr instead — the guest never stops.

Never cap a diagnostic log before you know the shape of the data. A probe limited to the first six transactions showed only 0xFF payloads, which supported exactly the wrong conclusion. Without the cap, the writes that mattered were obvious.

Check that the build succeeded before believing a test. A failed ninja leaves the previous binary in place and the test still runs, so a stale build silently answers the question. Two rounds of results were meaningless this way. Grep the build output for FAILED and error: and stop if either appears.

Reset the flash image between runs. assets/flash.img is written by every session. A test that starts from it may find its work already done — which shows up as "the image is byte-identical", indistinguishable from broken persistence. Start from assets/pristine/, and power the emulator off before restoring, since shutdown flushes the old in-memory image back over the file.

Do not hand-compute struct offsets. The ELF has no DWARF and the structs contain enums whose size cannot be assumed. Offsets computed by hand produced KEY_LOCK=4 and TX_VFO=11, neither of which is a possible value. Either use a symbol that nm reports and whose type is unambiguous (gInputBoxIndex is a plain uint8_t), or locate a field by behaviour — toggling the keypad lock with a long F press and diffing the region found KEY_LOCK at gEeprom+0x12 in one step.

Read your own probe output carefully. One probe printed phase before it was incremented, which made a correct address decoder look off by one byte. Replaying the logic in Python cleared it up; without that, a working implementation would have been "fixed".

Things that already went wrong

GDB breakpoints halt the guest. A key held across a breakpoint session is never processed, because the main loop is not running. This produced a whole round of "the keypress does nothing" that was really "the machine is stopped". Use tools/press_and_shot.sh — it presses, lets the machine run, then reads the framebuffer, with no breakpoints anywhere.

Do not write the SysTick counter back when accelerating it. Two attempts did that. Each read re-anchored the count, so the value the firmware saw stopped changing, its if (cur != prev) guard never fired, and the delay loop hung outright — worse than the slowness being fixed. The working approach reports a value that runs ahead of the real counter and leaves the timer alone.

Lowering the clock does not speed up delay loops. The bottleneck is loop iterations per second, not counter speed. 48 MHz to 200 Hz bought 32x and was nowhere near enough. Measured, not assumed.

Unnamed qdev in and out lines share one namespace. A device with both unnamed qdev_init_gpio_in and qdev_init_gpio_out makes qdev_get_gpio_in() ambiguous, and board wiring silently attaches to the wrong line. The GPIO model uses "pin-in" and "pin-out" for this reason. Keep it that way.

Key hold times must be SHORT, not generous. This entry used to say the opposite -- that guest time runs fast so a press needs a long hold, and that key.py should hold for 2500 ms. That was wrong and it broke the keypad tooling for a long time. 2500 ms is ~250 firmware ticks, six times past the long-press threshold, so every press was dispatched as a hold and handlers that act on a short release did nothing. See the keypad section below; key.py now holds 200 ms.

Verify a tool's own parsing before trusting its output. gpio_watch.py reported IDR=0x0000 for several rounds because its regex did not match gdb's output format at all. The register was fine; the reader was broken. Cross-check with tools/gpiob_dump.sh, which uses a different path.

QMP pmemsave is physical, memsave is virtual. The framebuffer symbols are CPU virtual addresses, so pmemsave on gFrameBuffer returns a block of zeros and reports success -- a blank screen with nothing logged anywhere. The web UI was built on pmemsave first because a timing benchmark said it was fast; the benchmark never checked the contents. Measure the thing you actually care about: the bug surfaced only when a rendered frame came back with 0 lit pixels where the gdb path reported 1693.

The keypad: two real bugs, both fixed

The old note here said "keys reach the firmware but the UI does not react" and blamed the machine model. There turned out to be two independent causes, in this order:

1. tools/key.py held every key for 2500 ms — a tooling bug, covered immediately below.
2. row_out was not volatile, so GCC deleted the row-driving code — a real model bug, introduced later while removing debug prints. See row_out must stay volatile.

Both are fixed and tools/keypad_test.py guards against regressions in either.

The two SysTick mechanisms are separate, and conflating them caused this:

• SysTick interrupts fire at close to real time. SysTick_Handler sets gNextTimeslice, which gates APP_TimeSlice10ms -> CheckKeys. So the debounce thresholds in App/misc.c apply in wall clock as written: key_debounce_10ms = 2 (20 ms to register), key_repeat_delay_10ms = 40 (400 ms counts as held).
• The poll-boost property accelerates SysTick counter reads, so SYSTICK_DelayUs converges. It does not speed up interrupt delivery.

A 2500 ms hold is ~250 ticks, six times past the long-press threshold. Every press was dispatched as a hold, and the handlers act on a short release: MAIN_Key_MENU returns early at the if (bKeyHeld) branch and never opens the menu. Confirmed by reading gDebounceCounter mid-hold — it stood at 317 after a 3 s hold, which both proves the timeslice is running and shows the hold was far too long.

Current values in key.py: HOLD_MS = 200, LONG_HOLD_MS = 900. Verified end to end — key.py MENU DOWN DOWN moves the menu from 01/79 to 03/79, and key.py UP moves it back to 02/79.

If a press seems ignored, do not lengthen the hold. Check whether the handler wanted a short press, and check gEeprom.KEY_LOCK (the LCD draws a padlock when the keypad is locked, and ignoring keys is then correct behaviour).

Driving the menus: send a sequence as one burst

Three things will make a key sequence land somewhere you did not intend. All three cost time here.

gdb between presses halts the guest. Every gdb-multiarch -batch attach stops the machine for its duration. Inspecting gMenuCursor after each press stretches a six-press sequence past the 20 s menu timeout (menu_timeout_500ms in App/misc.c), so the UI silently falls back to the main screen and the rest of the presses tune the VFO instead of navigating. Send the whole sequence in one Python burst over QMP, then read state once at the end.

UP/DOWN are inverted inside a submenu. MENU_Key_UP_DOWN flips Direction when gIsInSubMenu and !gEeprom.SET_NAV (app/menu.c:2311). In the list DOWN moves down; editing a value, UP decreases it. Values also clamp at MENU_GetLimits rather than wrapping, so overshooting sticks at the limit.

MENU toggles rather than only entering. On the main screen a short MENU opens the menu; in the list it enters the submenu; in a submenu it commits (gFlagAcceptSetting = true) and steps back out. Two MENU presses in a row from the list therefore enter and immediately leave, which looks like nothing happened.

Numeric jump: typing a menu number in the list jumps straight to it, which beats counting DOWN presses. Single digits are reliable. Two-digit entry needs both presses inside the same input-box window, and MENU_Key_0_to_9 jumps and returns as soon as the first digit is a valid index (app/menu.c:1826), so 3 then 0 lands on 3 rather than 30. Pre-positioning gMenuCursor with gdb, in one attach right after opening the menu, is the reliable way to reach a distant entry.

Verified this way: menu opens, DOWN/UP move the list, MENU enters a submenu, and a digit selects a value. Screenshots confirmed Step at 01/79, RxDCS at 03/79 after two DOWN presses, and BatSav at 30/79 showing OFF.

row_out must stay volatile or GCC deletes the keypad

UVK5KeypadState::row_out is declared qemu_irq volatile. Drop the volatile and the keypad stops working entirely: no press reaches the UI, awake or in power save, and nothing warns you. tools/keypad_test.py covers it.

The reason is visible in the object code. qdev_init_gpio_out_named() is inlinable and only records the array; the lines are filled in later by qdev_connect_gpio_out_named() from the board, which GCC cannot see. Left plain, GCC at -O2 proves every element is still NULL, sees that qemu_set_irq() returns immediately on a NULL irq, and deletes the body of keypad_update_rows() along with all five calls to it:

callers reaching keypad_update_rows
  plain     {}                     <- none; the calls are gone
  volatile  {keypad_key_changed, keypad_col_changed, keypad_set_press,
             keypad_reset, uvk5_machine_init}

keypad_col_changed compiles to a store and a ret with no call at all. With volatile it ends in jmp keypad_update_rows. So no row line is ever driven, the firmware's scan reads all-high, and the model looks broken.

Getting here took three wrong diagnoses, all worth knowing about:

1. "Power save stops the keypad scan." Written up here as a model gap. It was not: the breakage was present awake too.
2. "It needs settling time." Three fprintf(stderr, "TRACE ...") probes had been removed as cleanup, and restoring the one in keypad_update_rows fixed it, as did a busy loop in the same place. That looked like a timing dependency. It was not — the fprintf and the loop were just side effects GCC could not discard, which kept the loop alive.
3. "It is a compiler ordering problem." A zero-cost __asm__ __volatile__("" ::: "memory") also fixed it, 8/8. Same reason: a barrier is an unknown side effect, so the loop survives.

What settled it was comparing the two object files instead of the behaviour. The standalone keypad_update_rows symbol is instruction-identical either way, which is why an early diff of just that function found nothing — the function is inlined into its callers, and the difference is there.

Measurements, 3+ trials each, no debugger near the press:

variant	result
plain row_out	0/12
(void)r; added — inert, no side effect	0/6
identical rebuild (stability control)	0/6
busy loop, 1 to 4000 iterations	3/3
__asm__ ... "memory" barrier	12/12
volatile row_out (the actual fix)	10/10

Scope, checked rather than assumed: the other out-GPIO array in this file, PY32GpioState::out, is not affected. Marking it volatile as well produces a byte-identical object file, because the function that drives those lines (py32_gpio_write) is only reachable through a MemoryRegionOps function-pointer table, so GCC cannot do the whole-function reasoning that killed the keypad path. Leave it plain.

The general shape to watch for: a device whose out-GPIO lines are only ever connected from board code, driven from a function GCC can see all callers of. If a model's outputs mysteriously do nothing, check the object code for the call before assuming the logic is wrong:

objdump -dr build/libqemu-arm-softmmu.fa.p/hw_arm_py32f071.c.o \
    | grep -c qemu_set_irq

Two measurement mistakes made this much harder than it needed to be, both worth avoiding:

• Reading key state after releasing the key. gKeyReading0 is always KEY_INVALID once the key is up, so it "proves" the press was never seen. Read mid-hold instead.
• Trusting a gdb breakpoint on KEYBOARD_Poll. With the guest stopped the scan's delays cost no guest time, so Poll returns KEY_MENU under a breakpoint on a build where it returns KEY_INVALID when running free. That single observation sent this in the wrong direction for a long time.

Two related facts, both confirmed by experiment, so nobody spends time on them:

• Patching battery save in assets/flash.img does nothing. SETTINGS_InitEEPROM compares a version string at flash 0x00A160, finds a mismatch on a fresh image, and writes the settings sector. PY25Q16_WriteBuffer erases the whole 4 KB sector before reprogramming, so a byte planted at 0x00A00B is gone before the read at settings.c:169 sees it.
• Guest-side settings changes do not persist. The emulated PY25Q16 loads the image into RAM at realize time and never writes back, so anything the firmware saves is lost on restart. Adding a flush would be the fix if persistent settings are ever wanted. Nothing needs it today.

Useful here: tools/scan_trace.sh (what the scan reads), tools/key_result.sh (what Poll returns), tools/trace_run.sh (the TRACE points).

The three fprintf(stderr, "TRACE ...") probes that used to sit in qemu/py32f071.c are gone -- they fired on every keypad poll and buried the console. They went in py32_gpio_set_input, keypad_update_rows and keypad_col_changed; git log -p -- qemu/py32f071.c has the exact lines, and they are still the quickest way to see whether a press reaches the model (grep -c 'keypad row0 -> 0' on the captured stderr).

Redirect that stderr to a file rather than a pipe, and be aware that the keypad_update_rows one changes timing enough to matter -- see the settle-loop note above.

Note the ELF at uvk5-sat/build/CW/nr7y.cw.elf carries no DWARF, so gdb reports 'gEeprom' has unknown type. Scalars work if you cast through their address (*(unsigned short*)&gDebounceCounter); struct fields need manual offsets.

The BK4819, and where modelling it stops

The register interface is modelled (TYPE_UVK5_BK4819): the bit-banged three-wire bus is decoded, registers read back what the firmware wrote, and the ones it reads without writing return plausible values. Wiring is CS on PF9, SCL PB8, SDA PB9 with both directions connected. tools/test_bk4819.py inspects the register file over QOM.

This is what it fixed: RSSI used to read hard zero at 18 call sites — -160 dBm — so the S-meter showed empty and squelch and scan logic evaluated a dead band. The main screen now comes up on 400 MHz rather than the 18 MHz floor, because band setup is no longer reading zeros.

Two constraints are not negotiable, both from untimed spin loops in the firmware:

• REG_0C bit 0 must stay clear. app/app.c:910 and :1417 are while (BK4819_ReadRegister(BK4819_REG_0C) & 1u) with no timeout at all. A stuck bit hangs the guest; it does not degrade.
• A soft reset must re-seed the measurement registers. REG_00 bit 15, which BK4819_Init issues first, would otherwise leave them zero — real hardware keeps measuring. Not hypothetical: the first test run decoded 48 registers correctly and still reported RSSI as 0 for precisely this reason.

PTT, and the transmit level bar

PTT is not a matrix key. GPIO_IsPttPressed reads PB10 directly (driver/gpio.h:31, active low), so the model gives it its own GPIO line rather than a column/row intersection, exposed as a boolean ptt property on the keypad device.

That is what makes the transmit level bar reachable. app/app.c:1700 draws it only while gCurrentFunction == FUNCTION_TRANSMIT and gSetting_mic_bar is set — the latter is Data[7] bit 4 at flash 0xA0A8 (settings.c:423), and blank flash reads 0xFF, so it is already on. The level itself comes from REG_64 via BK4819_GetVoiceAmplitudeOut.

Treat the release as the important half. A stuck PTT leaves the emulated radio keyed, and every later test then runs against a transmitting radio. The web UI releases on pointerleave, pointercancel and pagehide; /api/release-all clears PTT explicitly, because an empty press does not touch it; and the endpoint rejects non-boolean bodies so {"held": "false"} cannot key the transmitter by truthiness. tools/test_ptt.py asserts the release, not just the press.

One trap worth knowing if you add another non-key button: the browser wired handlers over .key, which matched the PTT button as well, and it has no data-key — so it would have sent the key "undefined". Use .key[data-key].

Reads were shifted one bit, and it hid everything else

Fixed in ad88ee1, but worth reading because of how long it stayed invisible.

Register reads arrived shifted one place left: seed REG_0C with 0x1248 and the firmware received 0x2490. Each firmware bit is read/raise/lower, so the eighth command bit is followed by a falling edge before the data phase — and the model was treating that edge as a data clock, shifting bit 15 away before the guest sampled it.

Why nobody noticed: writes were always fine, 52 registers held exactly what the firmware wrote, and the register the firmware polls hardest was legitimately 0. Reading zero and getting zero looks like success. Verifying a read path requires a register with a known non-zero value — REG_3F is 0x0C0C, REG_78 is 0x2F5B.

tools/test_bk4819_readback.sh guards it now: seeds REG_0C (read ~1700 times per 30 s, so a sample is guaranteed) with a value carrying bits in both halves, and names the shift direction on failure. Bit 0 is left clear deliberately — with it set the firmware enters an untimed acknowledge loop, and that test is about alignment only.

This also invalidated four earlier diagnoses. Attempts at the squelch interrupt had the model raising REG_0C bit 0 while the firmware received bit 1, so

while (BK4819_ReadRegister(BK4819_REG_0C) & 1u)

was never true and 1719 polls saw a flag the guest could not act on. Every one of those rounds was blamed on timing or gating. When several independent attempts fail the same way, suspect the shared transport, not the logic on top of it.

The squelch interrupt and the S-meter: five attempts, then it worked

This works now (e6cebed) — skip to the end for the conclusion. The four failed attempts are kept because each produced a confident wrong diagnosis, and the pattern of how they failed is the useful part.

Scanning worked early on: long-press * and the frequency really does step, 6 distinct frames over 7 seconds. The S-meter did not, because ui/main.c:2370 only draws it when FUNCTION_IsRx(), and that needs gCurrentFunction in a receiving state — which takes the chip reporting a squelch opening, not just a healthy RSSI.

The mechanism looked clear: REG_0C bit 0 says an interrupt is pending, the firmware writes REG_02 to acknowledge and reads it back for the flags, and sqlFound is bit 3 (the bitfield is at app/app.c:915). Both the bit choice and that reading of the mechanism turned out to be wrong.

I implemented it — raise sqlFound once when the firmware enables interrupts — and backed it out. The guest kept running, but REG_0C bit 0 was still set afterwards: the firmware had not collected the interrupt. That is a latent hang, because app/app.c:910 and :1417 spin on that bit with no timeout, so any path that reaches them with the bit stuck never returns. Shipping a model that leaves a hang armed is worse than shipping one without an S-meter.

Second attempt, and the actual reason. Tried again, this time evaluating squelch when the firmware polls REG_0C rather than when it configures the chip — which fixed the original mistake, since the startup sequence writes REG_3F as 0x0000 then 0x0C0C three times over, so a flag raised on the enabling write was disabled again before anyone read it. Also corrected the threshold field: the RSSI open level is REG_78 bits 15:8 at 0.5 dB/step against REG_67's 0.25 dB/step, not anything in REG_4E (those low bits are the glitch threshold, and using them meant squelch never opened at all).

With that right, everything on the chip side lines up — measured en=0x0C0C, rssi=0x01E0, threshold 94, and REG_0C correctly returning 1. The firmware still never acknowledged. The reason is not on the chip side at all:

gCurrentFunction=5 (FUNCTION_POWER_SAVE), gRxIdleMode=1

and the gate is app/app.c:1697:

if (gCurrentFunction != FUNCTION_POWER_SAVE || !gRxIdleMode)
    CheckRadioInterrupts();

Both halves are false in that state, which looked like the answer: no CheckRadioInterrupts, so nothing to collect the flag.

That explanation is wrong, and the test that disproves it is worth keeping. app/app.c:1374 refuses power save outright when BATTERY_SAVE == 0, and the byte lives at flash 0xA00B (blank flash reads 0xFF, which settings.c clamps to 4 — the deepest setting, which is why the emulator idles there). Patch that byte to 0 and:

BATTERY_SAVE=4:  fn=5 idle=1   polls=2161  acks=0
BATTERY_SAVE=0:  fn=0 idle=0   polls=2161  acks=0

The gate now passes and the acknowledge count is still zero. A gdb backtrace on BK4819_ReadRegister confirms the loop really is running — CheckRadioInterrupts is inlined into APP_TimeSlice10ms, and that is the caller:

#0  BK4819_ReadRegister
#1  APP_TimeSlice10ms
#2  Main

So the firmware reads REG_0C, gets 1, and does not write REG_02. Whatever suppresses that is inside the inlined loop, past the gate. Gating the model on REG_30 (zeroed by BK4819_Sleep) does not help either — the chip is awake when the model is asked while the firmware still reports gRxIdleMode=1.

Resolved in e6cebed. The meter reads: -53 dBm, +40 over S9, nine of thirteen segments, MONI, and a running receive timer. The numbers agree — S9 is −93 dBm on UHF, so −53 really is S9+40.

Three things had to be right, and the order they were found in was the difficult part.

The flag is SQUELCH_LOST, bit 2. Per app/app.c:1027, "squelch lost" is what sets g_SquelchLost = true, meaning a signal is present. SQUELCH_FOUND reads like "found a signal" and means the opposite. Bit definitions are in App/driver/bk4819-regs.h:290.

Announcing has to be rate-limited — here every 64th poll. Announce once and the firmware collects it during startup, before the flag leads anywhere. Announce on every poll and the request bit is re-armed inside the firmware's own collection loop, which uses REG_0C as its condition and has no timeout, so it never exits. Periodic satisfies both: the loop always drains, and the news repeats until it matters.

The way in is not the interrupt at all. The radio idles in power save and does not act on squelch there — which is why a breakpoint on BK4819_GetRSSI never fired. ACTION_Monitor skips squelch entirely: app/app.c:482 picks FUNCTION_MONITOR over FUNCTION_RECEIVE whenever gMonitor is set, and settings.c:263 defaults an out-of-range stored action to ACTION_OPT_MONITOR — which blank flash (0xFF) is. So SIDE1 short-press engages monitor on a pristine image:

before:  fn=5 idle=1 monitor=0      (FUNCTION_POWER_SAVE)
after:   fn=2 idle=0 monitor=1

Gate on RX_DSP (REG_30 bit 0) rather than the whole register being zero: TX and tone paths leave other bits set with RX_DSP clear and would otherwise look like a live receiver.

tools/test_smeter.py covers the path end to end and compares lit-pixel counts rather than matching pixels, so an unrelated UI change cannot produce a mysterious failure.

Four measurement mistakes made this take far longer than the code involved. All four produced a confident, wrong conclusion:

• Sampling PC at the REG_0C read lands in BK4819_WriteU8, the bit-banging helper, not the caller. Sampling LR is no better: BK4819_ReadRegister calls BK4819_ReadU16, so LR points back inside the reader. Use a breakpoint and a backtrace.
• A probe printing shift_out before the assignment reported 0000 for a value about to be sent as 0001. Nearly became "the model sends the wrong value".
• BK4819_ReadRegister returning 0x0 for REG_0C looked like a broken read path, and I changed the bit timing on the strength of it. But REG_0C legitimately holds 0 in the committed build — there is nothing to raise it. A register read returning the register's actual contents is not evidence of anything. Check against a register the firmware demonstrably wrote (REG_3F is 0x0C0C, REG_78 is 0x2F5B).
• nexti after a breakpoint landed somewhere unrelated and reported r0 = 0, which fed the same wrong conclusion. finish gives the real return value.

Also note gdb cannot call guest functions on this target (print BK4819_ReadRegister(0x3f) errors out), and there is no gCurrentRSSI global to read — RSSI is used and discarded. Breakpoint plus finish is the only way to see what the firmware actually received.

Where it stops. This models the register interface, not the radio. It reproduces what the firmware commanded — frequency, power step, carrier keying in time — never the analogue result: keying envelopes, spurious emissions, sensitivity.

That is not a gap to close later. The chip has no public datasheet, so its driver is the only specification available, and a driver tells you which registers were written, never what left the antenna. Those questions need a real radio and a spectrum analyser. Do not let anyone conclude otherwise from a passing emulator test, including the one added here.

Timing is also deliberately wrong — see the SysTick section in README.md. Fine for menus and control flow; useless for signal timing.

Serial, both directions

Works, and tools/test_serial_rx.py proves it by speaking the real protocol: 0x0514 hello gets a 0x0515 ack, and 0x051B returns the requested EEPROM bytes. Attach with -serial unix:/path/to.sock or any other chardev; it defaults to serial0.

Three things had to line up, and each failed silently on its own:

• USART1 needs a chardev. It is otherwise a register stub with nowhere for incoming bytes to come from.
• DMA has to service USART, decrementing CNDTR. driver/uart.c never reads DR. It receives over a circular channel and locates new data with sizeof(UART_DMA_Buffer) - LL_DMA_GetDataLength(...), so a count that never moves means a buffer that always looks empty, no matter how many bytes arrived. The service runs on a CNDTR read, which is exactly where the driver looks — no timer needed, and nothing can be delivered before the guest asks for it.
• DR writes must also reach the chardev. They used to go only to stderr. A host tool would send a command, the firmware would answer, and the answer went somewhere the tool could not see. That is indistinguishable from being ignored, and it cost a debugging round: the first run of the new test reported "no reply at all" alongside zero bytes of boot output, which looked like broken receive when in fact transmit was fine and simply invisible.

Channels also record the length they were programmed with, because CNDTR counts down and the write offset has to come from the difference.

If you add a peripheral

1. Read the register layout from the CMSIS header
2. Model only what the firmware actually touches; the logging catch-all (py32-stub) shows you what that is
3. Watch for spin loops: any flag the firmware polls must be able to change, and write-1-to-start bits (like ADC_CR2_CAL) must never be stored set
4. Rebuild, run, and check with tools/where.sh that the firmware moved past where it used to stop